Tennis racquets are typically composed of a rigid frame which supports one or two continuous lengths of string woven into a pattern of main (vertical) and cross (horizontal) segments, with the ends of each length of string secured by a knot. Continuous one-piece knot secured stringing methods utilize a single continuous length of string that is used for weaving both mains and crosses, while continuous two-piece knot secured stringing methods require a separate length of string for the mains and the crosses.
During the racquet stringing process, string tension is set by an external tensioning device and the string segment is then clamped to temporarily retain the tension while the process is repeated for the next un-tensioned main or cross segment. The overall length of string is secured via the placement of knots at each of the free ends of the string.
The continuous knot secured stringing methods suffer from a number of disadvantages. As the disadvantages are similar for both the one and two-piece continuous knot secured methods, only the one-piece method will be discussed below. Since all of the main and cross segments are woven from the same single continuous length of string, any segment specific tensioning errors will tend to be communicated to all other segments that are necessarily composed of that length of string. As a result of this inter-segmental communication, successive tensioning errors will tend to combine in an additive manner and be distributed to some degree throughout the woven racquet face.
During the stringing process, common sources of tensioning error include: frictional forces and variations in these forces generated at the main and cross segment intersection points, frictional forces and variations in these forces generated at the pass-through points on the racket frame, and slippage of the strings within the temporary restraining clamps.
Furthermore, the knot tying process is inherently error prone in that the free end of the string must be removed from the tensioning device prior to the actual tying of the knot. In this case, once removed from the tensioning device, the string must be temporarily restrained without loss of tension while the final string routing and knot tying steps are performed. In practice, this is difficult to accomplish accurately and consistently. Consequently, with one or even two-piece knot secured stringing methods, it is difficult to accurately control and maintain tension over the entire length of the weave during the racquet stringing process.
Additionally, the continuous nature of the weave imposes serious limitations on segment specific tension variability. In the most restrictive case, the one-piece continuous knot secured method necessarily requires that every segment in the weave have the same tension. The two-piece continuous knot secured method is slightly less restrictive in that it allows the mains and the crosses to be independently tensioned. In either case, there is little opportunity for racquet users or designers to explore the potential benefits of segment specific tensioning.
The continuous nature of the weave also imposes serious limitations on segment specific string composition. In the most restrictive case, the one-piece continuous knot secured method necessarily requires that every segment in the weave be composed of the same string type and size. The two-piece continuous knot secured method is slightly less restrictive in that it allows the mains and the crosses to be composed of two string types and sizes. In either case, there is little opportunity for racquet users or designers to explore the potential playability and durability benefits of completely independent segment string types and sizes.
Tensioning of each segment requires that the free end of the string be placed in the gripper or gripping jaw of a tensioning device. Once the string is secured in the gripping jaw, the tensioning device is engaged thus applying tension to the length of string contained therein. In a one-piece continuous knot secured stringing method, nearly all incidental abrasions inflicted upon the string by the gripping jaws of the tensioning device will be incorporated into the body of the weave at some point during the stringing process. These abraded areas of the string are points of stress concentration and are consequently more likely to experience a breaking failure.
Furthermore, as all of the main and cross segments are woven from the same single length of string, the failure of any single main or cross segment requires that all segments be replaced. This is tremendously inefficient with regards to both the time and the materials required to re-string a racquet.
It is known that U.S. Pat. Nos. 4,309,033 and 4,333,649 disclose a wedge shaped member which is disposed between the racquet frame and the length of string passing therethrough, so as to engage and secure the string. However, the approach taken in these prior art disclosures necessarily requires the wedge member to engage and begin gripping the string segment prior to final positioning of the wedge within the frame passageway, and consequently prior to final positioning of the wedge with respect to the string segment. This early or premature engagement of the gripping wedge may lead to scoring or abrasion of the string surface, which may ultimately weaken the string and result in a breaking failure under playing conditions. Furthermore, premature engagement of the wedge member also necessarily interferes with the ability to accurately maintain the string segment at a predetermined tension during the stringing process. That is, as the wedge is inserted into the passageway of the frame and begins to prematurely engage the string segment therein, the portion of the string being engaged by the wedge will tend to be pulled in the direction of movement of the gripping wedge. Obviously, any pulling or pushing force applied along the axis of the string segment during the securing process will necessarily affect the tension of that string segment.
Additionally, the wedged string clamping devices described in the above mentioned prior art, have no way of preventing the gripping wedge from backing out of passageway once insertion is complete and final positioning of the wedge has been accomplished. Any tendency of the wedge member to back out of the frame passageway once final positioning has been achieved, will necessarily result in a loss of tension in the secured segment of string.
Therefore, there is and continues to be a need for a practical and economical stringed sports racquet wherein each main and cross segment of the woven racquet face can be comprised of a separate, independent length of string and whereby each string segment can be quickly, accurately and independently tensioned and secured.